In heavy vehicles such as military tanks which are subject to extreme environmental conditions the engines can be difficult to start, thus causing the battery voltage to drop at least momentarily to very low levels during starting. Since sustained low voltage operation of the starter motor can be injurious to the motor, it is the practice to provide a protection circuit for the starter motor which monitors the system voltage when the starter is first energized and disables the motor if the voltage remains below a threshold for a prescribed time period.
The protection circuit includes a relay having switching contacts in series with the starter motor. The relay coil is subject to the highly variable system voltage and must be operable throughout the voltage range. However, a coil which is selected for low voltage operation is vulnerable to being burned out if a sustained high voltage is applied. Thus it is desirable to provide some form of protection to avoid the effects of high voltage on the relay coil.
As discussed in the U.S. Pat. No. 4,585,986 to Dyer, entitled "DC Switching Regulated Power Supply For Driving an Inductive Load", it is known to use a switching control method in the art of automatic regulation of DC power to a load. The average power supplied to a load is regulated by a series switch which is operated to chop the current flowing to the load. Control is effected by varying the relative on time or duty cycle which is determined by comparing the load voltage to a reference voltage. In the present invention, it is recognized that regulation akin to that mentioned by Dyer could be applied to the high voltage protection of a relay coil in a starter protection circuit.